A Submersible Holographic Microscope

A new device allows scientists to form 3D images of tiny marine organisms at depths as great as 100 m. The device allows the recording of behavioral characteristics of zooplankton and other marine organisms in their natural environment without having to bring specimens to the surface for examination.

Scientists at Dalhousie University in Halifax, Canada, used the hologram arrangement originally invented by Denis Gabor: light from a laser is focused on a pinhole that acts as a point source of light if the size of the hole is comparable to the wavelength of light. The spherical waves that emanate from the pinhole illuminate a sample of sea water. Waves scattered by objects in the sea water then combine at the chip of a CCD camera with un-scattered waves (the reference wave) from the pin hole to form a digitized interference pattern or hologram. The digital holograms are then sent to a computer where they are digitally reconstructed with specially developed software to provide images of the objects.

The Dalhousie researchers packaged their holography apparatus in such a way that the laser and digital camera parts are in separate watertight containers, while the object plane is left open (see images at Physics News Graphics).

One difficulty was to get container windows of optical quality that are thin enough for high resolution imaging but thick enough to resist sea pressure. The new submersible microscope can also record the trajectories of organisms in the sample volume so that movies of the swimming characteristics of micron-sized marine organisms can easily be produced.

Holograms with 1024x1024 pixels can be recorded at 7 to 10 frames per second. This requires a large bandwidth for data transmission to a surface vessel and was accomplished with water tight Ethernet cables. Imaging volumes can be several cubic centimeters depending on the desired resolution.

The Gabor geometry allowed the Dalhousie researchers to design a very simple instrument capable of wavelength limited resolution of marine organisms in their natural environment. Past generations of submersible holographic microscopes had lower resolution, weighed several tons, had to be deployed from large ships, and used high-resolution film as the hologram recording medium. This meant that only a small number of holograms could be recorded. In contrast, the Dalhousie instrument only weighs 20 kilograms, or 44 pounds, can be deployed from small boats or even pleasure vessels, and can record thousands of holograms in a few minutes so that the motion of aquatic organisms can be captured in detail.

Jericho et al., Review of Scientific Instruments, upcoming article
Contact M.H. Jericho, jericho@fizz.phys.dal.ca
Images at Physics News Graphics